It seems to me it would be easy to zero in on the first group to investigate based on who stayed open and who closed on May 1st.
Round 'em up:
"Tyson Foods, the world's largest meat producer, shut 5 of 9 beef plants and 4 of 6 pork plants in anticipation of widespread absences. Perdue Farms, the nation's third-largest chicken producer, closed eight processing plants in seven states. Cargill Meat Solutions, the nation's second-largest beef processor, gave more than 15,000 workers the day off and closed plants in six states."
http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2006/05/02/BUGE2IIOT01.DTL&hw=cargill&sn=003&sc=980